The present invention relates to overhead projectors (OHP). In particular, the present invention relates to an OHP having an adjustable cantilevered head having a friction-controlled position adjustment mechanism. The cantilevered projection head design of the present invention provides sufficient force on friction pads to support the arm/head assembly. A spring-loaded pad is added only to prevent chatter of the arm/head assembly during user adjustment.
Overhead projectors project a transparency placed on a stage onto a screen. Traditional overhead projectors generally include a lighted horizontal stage and a projection head including optical components suspended above the stage by an arm coupled to a vertical post. The projection stage generally includes further optical components, such as a Fresnel lens. The distance between the projection head and the stage controls the focus of the projected image by controlling the distance between optical components on the head and on the stage.
As most overhead projectors are not consistently placed at the same fixed distance from the screen, the projection head/arm assembly has a range of travel along the post. Both gross control and fine head position control are required, first to move the head to a general position and then to tune the sharpness of the focus on the screen. Once the image focus is tuned, the projector head should remain in the exact registration chosen.
Different overhead projectors use a variety of mechanisms for providing both the gross and fine position control required. A basic OHP design uses a pin and cavity, or other interactive locking arrangement, to lock the head along one of various predetermined stop positions along the post. While this method offers rapid gross adjustment, fine tuning is limited due to the finite number of predetermined stops.
Other OHP designs, such as a 3M model 9050 OHP, use a spring loaded mount design that relies on friction pads. This type of design is discussed in U.S. Pat. No. 5,245,370, assigned to Minnesota Mining and Manufacturing Co. FIG. 1 generally illustrates the retention mechanism of this design, which will be referred to as the two-spring design. The design 10 includes a post 12 and an arm 14. The proximal end of arm 14 includes collar 16, a hollow casing that fits around the post 12. The combined weight of the head (not shown) and of the arm assembly is illustrated as a single force W placed at the center of mass. Inside the collar 16, four friction points 20, 22, 24, and 26, retain the arm 14 onto the post 12. In reality, each friction point may include one or more pads. The present text illustrates friction points that represent the summation of the forces applied at that point.
The two-spring design is advantageous because the head/arm assembly may be placed at any point along the travel on the post 12 and is not limited to a number of finite preset positions. However, implementation of the two-spring design requires mechanical complexity in the form of added spring-loaded mechanisms. The part count and assembly of such an arrangement is high. Furthermore, the present invention analyzes how the spring mechanisms rely on unnecessary and contradictory forces that overconstrain the system. This unnecessary constraints must be overcome to move the arm/head, requiring added complexity in adjustment systems.
To move the arm/head assembly, a rotating cog or gear on the head interacts with a column of gear teeth mounted on the post. The user rotates the cog by turning a knob. Since the knob must overcome considerable friction (from the overconstraint discussed below) and must not release independently, the resistance of the knob is usually set to require a moderate to heavy twisting force to be advanced, which places strain on the user. Therefore, gross adjustments are tedious and require some amount of time and effort from the user.
The need remains for an OHP having a user-friendly head/arm assembly adjustment system. Preferably, such a system would offer ease of assembly and manufacture, cost-effectiveness, and reliability.
The present invention is directed to an overhead projector comprising a stage assembly and a post extending from the stage assembly. The stage assembly includes a main body and a stage, generally in the form of a glass and Fresnel lens stack.
The OHP further includes an optical projection head assembly suspended above the stage assembly by the post in a cantilever arrangement, the optical head assembly having a weight W. The head assembly includes a post retaining assembly having a collar fitted around the post, the collar having at least two friction points interacting with the post, the interaction having a coefficient of friction xcexc. The weight W and the cantilever arrangement create torsional forces, the at least two friction points being positioned to transmit the torsional forces onto the post creating a friction force Ff.
The coefficient of friction of the interaction between each of the at least two friction points and the post is selected such that absent other forces the friction force is at least sufficient to maintain the head assembly at a static position with respect to the post.
That is, the coefficient of friction of each interaction is selected such that the total friction force Ff meets the following condition:
Ffxe2x89xa7W.
In an embodiment, the at least two friction points include two friction pads, wherein the two friction pads are each positioned on opposite sides of the post and are spaced from each other a vertical distance L. Where the friction points have the same friction coefficient, L and xcexc are selected such that       F    f    =            μ      ⁢              xe2x80x83            ⁢              (                  W          ⁢                      xe2x80x83                    ⁢                                                    (                                                      2                    ⁢                                          xe2x80x83                                        ⁢                    D                                    +                  d                                )                            ⁢                              xe2x80x83                            ⁢              L                                                      L                2                            +                              d                2                                                    )              ≥    W  
Where the friction points have different friction coefficients xcexcp1 and xcexcp2, L and values of the friction coefficients are selected such that       F    f    =            [                                    μ            p1                    ⁢                      xe2x80x83                    ⁢                      (                          W              ⁢                              xe2x80x83                            ⁢                              DL                                                      L                    2                                    +                                      d                    2                                                                        )                          +                              μ            p2                    ⁢                      xe2x80x83                    ⁢                      (                          W              ⁢                              xe2x80x83                            ⁢                                                                    (                                          D                      +                      d                                        )                                    ⁢                                      xe2x80x83                                    ⁢                  L                                                                      L                    2                                    +                                      d                    2                                                                        )                              ]        ≥    W  
In one embodiment, at least one of the friction points may be removed from contact with the post to allow movement of the head assembly to a different position along the post. An adjustment mechanism allows the user to move the head assembly to a different position along the post comprises a gear and rack system. Alternatively, the adjustment mechanism that allows the user to move the head assembly to a different position along the post comprises a friction mechanism.
In yet another embodiment, the adjustment mechanism allows the user to move the head assembly to a different position along the post, wherein the torsional force on at least one of the friction points is reduced during movement. The friction mechanism may be mounted on at least one of the friction points.
The overhead projector may further comprise a spring that loads at least one of the existing friction points in the same rotational direction as the torsional force created by the weight W thereby increasing the frictional force. Where the friction points have different friction coefficients xcexcp1 and xcexcp2, L and values of the friction coefficients are selected such that       F    f    =                    μ        p1            ⁢              xe2x80x83            ⁢              (                              W            ⁢                          xe2x80x83                        ⁢                          DL                                                L                  2                                +                                  d                  2                                                              +                                    F              s                        ·                                                            (                                      L                    +                    h                                    )                                ⁢                                  xe2x80x83                                ⁢                L                                                              L                  2                                +                                  d                  2                                                                    )              +                  μ        p2            ⁢              xe2x80x83            ⁢              (                              W            ⁢                          xe2x80x83                        ⁢                                                            (                                      D                    +                    d                                    )                                ⁢                                  xe2x80x83                                ⁢                L                                                              L                  2                                +                                  d                  2                                                              +                                    F              s                        ·                          hL                                                L                  2                                +                                  d                  2                                                                    )            
where
Ffxe2x89xa7W.
The spring may be used to load an additional friction point. Then, where the friction points have different friction coefficients xcexcp1 and xcexcp2 and the additional friction point has a coefficient of friction xcexcs, L and the values of the friction coefficients are selected such that       F    f    =                    μ        p1            ⁢              xe2x80x83            ⁢              (                              W            ⁢                          DL                                                L                  2                                +                                  d                  2                                                              +                                    F              s                        ·                                                            (                                      L                    +                    h                                    )                                ⁢                                  xe2x80x83                                ⁢                L                                                              L                  2                                +                                  d                  2                                                                    )              +                  μ        p2            ⁢              xe2x80x83            ⁢              (                              W            ⁢                          xe2x80x83                        ⁢                                                            (                                      D                    +                    d                                    )                                ⁢                                  xe2x80x83                                ⁢                L                                                              L                  2                                +                                  d                  2                                                              +                                    F              s                        ·                          hL                                                L                  2                                +                                  d                  2                                                                    )              +                  μ        s            ·              F        s            
The at least two friction points comprise two friction pads placed near opposite ends and at opposite sides of the collar, and the spring loaded friction pad is placed in a location not between the friction pads.
The present invention also contemplates an adjustable attachment mechanism for an overhead projector, the overhead projector comprising a post extending from a main body supporting a cantilevered weight W. The attachment mechanism includes a post retaining assembly having a collar fitted around the post, the collar having at least two friction points interacting with the post, the interaction having a coefficient of friction. The weight W and the cantilever arrangement create torsional forces, the at least two friction points being positioned to transmit the torsional forces onto the post creating a friction force Ff. The coefficient of friction of the interaction between each of the at least two friction points and the post is selected such that absent other forces the friction force is at least sufficient to maintain the head assembly at a static position with respect to the post.